1. Field of the Invention
2. Description of the Related Art
Recently, flat-panel type display devices in which pixels (pixel circuits) are arranged in the form of a matrix have spread rapidly in a field of display devices displaying images. As one of the flat-panel type display devices, there is a display device using a current-driven type electrooptic element whose light emission luminance changes according to the value of a current flowing through the device as light emitting elements of pixels. As a current-driven type electrooptic element, an organic EL (Electro Luminescence) element utilizing a phenomenon of light being emitted when an electric field is applied to an organic thin film is known.
An organic EL display device using the organic EL element as electrooptic elements of pixels has the following features. The organic EL element can be driven by an application voltage of 10 V or lower, and thus consumes low power. Because the organic EL element is a self-luminous element, as compared with a liquid crystal display device that displays an image by controlling the intensity of light from a light source in a liquid crystal in each pixel, the organic EL display device provides high image visibility, and is easily reduced in weight and thickness because an illuminating member such as a backlight or the like is not required. Further, because the organic EL element has a very high response speed of a few μsec or so, no afterimage occurs at a time of displaying a moving image.
As with the liquid crystal display device, the organic EL display device can adopt a simple (passive) matrix system and an active matrix system as a driving system of the organic EL display device. However, while having a simple structure, a simple matrix type display device presents for example a problem of difficulty in realizing a large and high-definition display device because the emission period of an electrooptic element is reduced by an increase in the number of scanning lines (that is, the number of pixels).
Therefore an active matrix type display device that controls current flowing through an electrooptic element by an active element, for example an insulated gate field effect transistor provided within a same pixel circuit as the electrooptic element has recently been actively developed. A TFT (Thin Film Transistor) is typically used as the insulated gate field effect transistor. The active matrix type display device makes it easy to realize a large and high-definition display device because the electrooptic element continues emitting light over the period of one frame.
It is generally known that the I-V characteristic (current-voltage characteristic) of the organic EL element is degraded with the passage of time (so-called secular degradation). In a pixel circuit using an N-channel type TFT in particular as a transistor that current-drives an organic EL element (which transistor will hereinafter be described as a “driving transistor”), when the I-V characteristic of the organic EL element is degraded with the passage of time, the gate-to-source voltage Vgs of the driving transistor changes. As a result, the light emission luminance of the organic EL element changes. This is caused by the connection of the organic EL element to the source electrode side of the driving transistor.
This will be described more specifically. The source potential of the driving transistor is determined by an operating point of the driving transistor and the organic EL element. When the I-V characteristic of the organic EL element is degraded, the operating point of the driving transistor and the organic EL element varies. Thus, even when a same voltage is applied to the gate of the driving transistor, the source potential of the driving transistor changes. Thereby, the gate-to-source voltage Vgs of the driving transistor changes, and therefore the value of current flowing through the driving transistor changes. As a result, the value of current flowing through the organic EL element also changes, so that the light emission luminance of the organic EL element changes.
Further, in a pixel circuit using a polysilicon TFT, in addition to a secular degradation in the I-V characteristic of an organic EL element, transistor characteristics of a driving transistor vary with the passage of time, and the transistor characteristics vary from pixel to pixel due to variations in a manufacturing process. That is, the transistor characteristics of the driving transistor vary between individual pixels. The transistor characteristics include for example the threshold voltage Vth of the driving transistor and the mobility μ of a semiconductor thin film forming the channel of the driving transistor (which mobility μ will hereinafter be described simply as the “mobility μ of the driving transistor”).
When the transistor characteristics of the driving transistor differ in each pixel, the value of current flowing through the driving transistor varies in each pixel. Thus, even when a same voltage is applied to the gates of driving transistors in respective pixels, the light emission luminance of the organic EL element varies between the pixels. As a result, screen uniformity is impaired.
Accordingly, in order to hold the light emission luminance of the organic EL element constant without being affected by secular degradation in the I-V characteristic of the organic EL element or secular changes in the transistor characteristics of the driving transistor, for example, the pixel circuit is provided with various correcting (compensating) functions (see Japanese Patent Laid-Open No. 2006-133542, for example).
The correcting functions include for example a function of compensating for variations in the characteristic of the organic EL element, a function of correcting for variations in the threshold voltage Vth of the driving transistor, and a function of correcting for variations in the mobility μ of the driving transistor. Hereinafter, correction for variations in the threshold voltage Vth of the driving transistor will be referred to as “threshold value correction,” and correction for variations in the mobility μ of the driving transistor will be referred to as “mobility correction.”
By thus providing each pixel circuit with the various functions, the light emission luminance of the organic EL element can be held constant without being affected by secular degradation in the I-V characteristic of the organic EL element or secular change in the transistor characteristics of the driving transistor. As a result, the display quality of the organic EL display device can be improved.